Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more co-axial cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Such rotary hammers can also be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forwardmost part thereof to rotate.
Such rotary hammers have a mode change arrangement for switching the hammer between hammer only mode, rotary hammer mode and/or drilling only mode. Preferably, the mode change arrangement is operated by a single mode change knob which is used to switch between all of the modes of the hammer. Because the mode change knob must selectively engage the rotary drive to the spindle and must also selectively engage the hammer drive to the air cushion hammering mechanism, the mode change arrangement may become quite complex. This problem is exacerbated in that the rotary drive and/or the hammer drive can be located within different spaced apart portions of the hammer. Also, it is desirable from an ergonomic point of view to locate the mode change knob towards the rearward end of the gearbox casing of the hammer, so that it is easily reached by an operator of the hammer. The mode change arrangement must withstand a high level of vibration and still accurately switch between modes over the lifetime of the hammer and so a robust design of mode change arrangement is required. Finally, in smaller rotary hammers, in particular those having a wobble plate hammer drive, there are tight space constraints and so the mode change arrangement must be relatively compact.
The present invention aims to provide a rotary hammer with a compact, robust and reliable mode change arrangement for switching between the modes of the hammer.